Certain types of glass structures such as glass sheets are fabricated using a lamination process that combines different layers of different types of glass. The lamination process can be such that the interfaces between the different glass layers become gradual. This can happen, for example, if the lamination process involves heat that gives rise to thermal diffusion that causes a gradient-index transition region between adjacent layers within the glass structure.
A gradual transition region between adjacent glass layers makes it difficult to use conventional methods for measuring glass-layer thicknesses that rely on a reflection from a relatively well-defined (i.e., sharp) internal interface. This is because the reflection of light from a gradually changing interface is typically too weak to be clearly detected. Further, modern-day glass-sheet manufacturing requires tight thickness control of each glass layer in the laminated glass structure. Such thickness control can require, for certain applications (e.g., screens for liquid crystal displays), a thickness measurement resolution of just a few microns. Such a measurement resolution cannot be obtained for glass structures having gradual transition regions between adjacent layers using the aforementioned conventional measuring techniques that rely upon reflection from a sharp interface between adjacent layers.